1. Field of the Invention
The present invention relates to a gasket for layer-built fuel cells. More precisely, the present invention relates to a gasket, for a sheet component of the layer-built fuel cells, having an excellent sealability when used in the layer-built fuel cells. Moreover the present invention relates to a gasket having an excellent processability when the layer-built fuel cell s is assembled.
Still more, the present invention relates to a method for making a gasket, for a sheet component of the layer-built fuel cells, having an excellent sealability when used in the layer-built fuel cells.
2. Description of the Related Art
Fuel cells, electrochemical device for continuously converting chemicalsxe2x80x94a fuel (hydrogen) and an oxydant (oxygen)xe2x80x94into direct current electricity, have electronic-conductor electrodes on which electrochemical reactions are taking place. The electrodesxe2x80x94a fuel electrode and an oxygen electrodexe2x80x94are usually coated with a fine powder of platinum-based catalysts thereon. Generation of electricity is carried out through electrochemical reactions which differ from a conventional oxidation reactions.
As for electrodes of the fuel cells, materials having a high electronic conductivity, an excellent stability for electrochemical reactions and an excellent anti-degradation property for electrolytes such as phosphoric acid or the like, have been used so far. For example, an amorphous carbon, or a powder or a fiber made of graphite, has been used conventionally. Especially, electrodes made of carbon easily accept electrons from hydrogen, a fuel, when hydrogen is ionized during electrode reactions. The electron accepted by electrodes becomes electric current and electric voltage. Carbon electrodes have also high electric conductivity and facilitate ionization of hydrogen in the electrode reaction.
Since reactions on the electrodes are carried out by contact processes between gas (a fuel) and a solid (electrode), the surface of the electrodes is required to be as large as possible for securing the speed of the reaction. This means that the electrodes should have porous structure in its surface portion to secure large contact surface area between gas and the electrodes. Carbon having excellent electron conductivity is generally stiff and brittle so that the physical strength of the carbon plate having porous structure (porosity is in the range of from about 40% to about 70%) tends to become low. These carbon plates are used in fuel cells as a current-collecting electrode (separator), a reaction electrode or the like. Fuel cells are built up using these sheet-like components such as the current-collecting electrode, the reaction electrode and an ion-exchange membrane.
Moreover, fuel cells are required to be sealed so as not to leak fuel gas (hydrogen, oxygen or the like) and liquid (liquid electrolyte or water produced in the electrochemical reaction) from the fuel cells in order to secure efficiency of power generation, longevity and stability of the device or the like. Especially, these fuel gasses or liquids tend to leak or permeate out of the fuel cells at the periphery of the carbon plates or the ion-exchange membranes.
In order to prevent liquid or gas from leaking, various seals such as gaskets (Japanese unexamined patent 9-231987, 7-263004, 7-226220 and 7-153480), rubber plates with cellular rubber layer thereon which are used as gaskets (Japanese unexamined patent 7-312223), have conventionally been used.
As is shown in FIG. 13 which is one of prior arts, there are several components such as a current-collecting electrode (separator) 2, an ion-exchange membranes 3 interposed between a membrane-fixed reaction electrode 4, a gasket or the like. The gasket is made of a cellular rubber 6 and a rubber plate. But these prior arts did not aim at seeking seals having thin-wall, good processability in building up fuel cells, a property hard to displace from its initial sealing place, good sealability even at low sealing pressure, uniformity of sealing pressure around its circumference or the like. That is, conventional gaskets, which are not unitized with sheet-like components, can not satisfactorily provide seals with thin-wall, good processability in building up the fuel cells, a property hard to displace from its initial sealing place or the like.
What is more, fuel cells have usually hundreds of carbon electrodes and sheets (separators and ion-exchange membranes) having a thickness of from about 0.1 mm to a few mm and one side dimension of from about 10 cm to about 50 cm in square or rectangular shape. Each sheet is so thin that it tends to get wrinkled. And carbon sheets have undulation in itself. From these causes, leaks of gas or liquid from the fuel cells have often been observed at the conventional sealing portions due to displacement or uneven pressure in sheets or electrodes. There have also been a big problem concerning processability in stacking each sheet to its correct position.
Materials made of polytetrafluoroethylene resin (PTFE resin) have been generally used as a seal for a phosphoric acid type fuel cells which is now regarded as a most promising one. Polytetrafluoroethylene resin can be used as a sealing material under such a severe condition as the presence of concentrated phosphoric acid at about 200xc2x0 C. in the phosphoric acid type fuel cells. Seals made of polytetrafluoroethylene resin are, however, hard in hardness so that sealing pressure must be higher to get a good sealing condition. But high sealing pressure causes breakdown of the carbon electrodes because the carbon electrodes are brittle due to its porosity mentioned above. And seals made of polytetrafluoroethylene resin have also such a shortcoming as difficulty in processing to make the seal. Ion-exchange membranes are also brittle when dried, so that strong stress added on the membranes may cause damage of the membranes.
Seals made of polytetrafluoroethylene have an excellent resistance against heat and chemicals in practical use as mentioned above, but can not completely seal the surface of the porous carbon plate because there is a convexo-concave portion on the surface of the electrode. Therefore seals made of polytetrafluoroethylene resin have been used with accepting a certain amount of leaking of fuel gasses, or used on carbon electrodes which are burnt after flattened and smoothed by coating on the surface of the carbon electrodes with polytetrafluoroethylene aqueous dispersion or the like, and then seals are pressed onto the polytetrafluoroethylene-coated carbon electrodes (Japan Unexamined Patent 58-078372, 59-068171). Therefore there have been some troubles such as time-consuming in process of manufacturing seals, instability in sealing property and difficulty in making thin-walled sealing members.
An object of the present invention is to provide a gasket, which is used for layer-built fuel cells, the gasket being low in height to allow the layer-built fuel cells to be more compact in size and showing an excellent sealing property even at a low sealing pressure. Another object of the invention is to provide a gasket, which has an excellent processability when the layer-built fuel cells are assembled.
Still another object of the present invention is to provide a method for manufacturing a gasket having no weld portion in itself, having an excellent sealability, and being used for a layer-built fuel cell.
The object of the present invention can be achieved by a gasket which is formed on a sheet component of fuel cells, and made of a liquid rubber vulcanizates having a hardness preferably not more than 60 (JIS A of JIS K 6301), the vulcanizates being unitized with the sheet.
Liquid rubbers used in the invention have a viscosity not more than 104 Paxc2x7s (at 25xc2x0 C.). Liquid perfluoro rubber is preferably used in this invention because perfluoro rubber has an excellent heat resistance and chemical resistance. Moreover a polytetrafluoroethylene fine powder can preferably be added into the liquid perfluoro-rubber for decreasing gas permeability. When porous carbon plates made of fibrous carbon are used as the sheets, liquid rubbers are infiltrated into the porous carbon plates, thereby the infiltrated portions become non-permeate portions against gas or aqueous electrolyte or water produced via electrochemical reactions, and become gasket portions too.
When porous carbon plates made of carbon other than the fibrous carbon are used as the sheets, the sealing portion (gasket) is made, integrally with the porous carbon plates, on the surface of the porous carbon plate or on a groove which is formed on a surface of the porous carbon plate.
When ion-exchange membranes are used as the sheets, the sealing portion is made, integrally with the ion-exchange membrane.
The sealing portions of these cases have preferably a hardness (JIS A) not more than 60 and are made of various liquid rubber having a viscosity not more than 104 Paxc2x7s (at 25xc2x0 C.).
Another object of the present invention can be achieved by a process of an injection-molding of liquid rubbers under vacuum circumstances. The molding processes of the present invention is as follows; first sucking out air in a closed space between an upper and lower mold, then closing the molds and injecting liquid rubber on a predetermined place of the surface of the porous sheet, and then curing the rubber in a predetermined period of time. There is no weld portion because no air is substantially in a cavity of the mold.
These features and advantages of the inventions will become apparent from the following description.